The invention relates to an apparatus and a method for separating particles of material of different electrical conductance. More specifically, the mixed particles are fed onto a conveyor and separated by a rotating magnetic system arranged on the conveyor. A collecting container collects the sorted particles which are separated from the remaining material on the conveyor.
In an apparatus of this type known from U.S. Pat. No. 3,448,857, a quantity of particles of different electrical conductance, which have to be sorted, is fed onto a conveyor belt from above. The conveyor belt turns over a belt drum and takes the particles to that drum at a speed of 1 to 1.5 m/sec. A magnetic system rotates in the belt drum at a speed of approximately 1500 r.p.m. A relative movement takes place between the conveyor belt and the drum with the magnetic system during operation, and the difference in speed makes the magnetic lines of force cut through the electrically conductive particles travelling on the belt. Currents are thereby induced of a strength dependent on the electrical conductance of the particles. A stronger current is generated in the particles with higher electrical conductance, and causes those particles to be thrown in a trajectory from the belt into their direction of movement. The particles with lower electrical conductance remain near the belt and drop off it almost vertically. The exact fraction which has a certain required electrical conductance may be filtered out by installing a collecting container at a suitable location. It should be noted that before the '857 apparatus is used, ferromagnetic materials have already been picked out of the material to be sorted by methods which are sufficiently well known (strong magnets). The main function of the '857 apparatus is in fact to separate so-called nonferrous metals (copper, aluminum, lead, zinc, brass etc) from residual materials (paper, plastic, glass etc) particularly in connection with waste recycling.
An apparatus for separating mixtures of materials with different electrical conductance including a similar rotating magnetic means is known from DE 34 16 504 A1. In this apparatus, the magnetic means rotates rapidly and produces a changing magnetic field through which the mixed particles are passed. The separating means is surrounded by a case which rotates more slowly. The eddy currents arising have effects on the particles, giving the electrically conductive particles a greater trajectory than the non-conductive ones.
PCT Patent Publication No. WO 89/07981 shows a comparable construction. Here again materials made up of non-magnetic particles drop from above onto a rotating drum containing a magnetic system which also rotates. The drum and the magnetic system rotate in opposite directions so that non-metallic materials such as glass, plastic and stones drop down on one side of the drum and non-magnetic metals on the other side. However, constructions in accordance with DE 34 16 504 A1 or WO 89/07981 only allow non-specific separation, and the number of incorrectly separated particles is relatively high. Magnetic particles which have not been previously separated out also pose a problem, and may cause damage on going between the drums and the cases rotating in the other direction.
As a further improvement to such apparatus EP 0 339 195 BI proposes that the magnetic system should be arranged eccentrically in the belt drum. This arrangement prevents magnetisable, electrically conductive particles from being held between the conveyor belt and the belt drum. These trapped particles become red hot as a result of the magnetic field and inflict corresponding damage to the belt drum and conveyor belt. Another eccentric arrangement is shown in Japanese Patent Publication JP57-119856 A.
It has already been proposed in German Patent Publication DE 4 323 932 C1 that the speed of the drum of the magnetic system should be raised and the deflection thus made stronger in order to improve sorting quality. However, this necessitates correspondingly expensive improvement of the properties of the magnetic system.